Returning to work/Using all 12 weeks of FMLA

It's time, or getting close to time for most of us January mom's to get back to work. 😩 My 8 weeks is up on March 5th but I'm not ready to go back by any means..physically or emotionally. I want to take the full 12 weeks of FMLA I'm allowed. My post partum check up is this week and I want to know if I have to get my OB to fill out anything additional in order for me to do this? Do I have to have a legit medical reason to take 12 weeks instead of 8? I didn't go back to work for over 2 years with my first 7 years ago so I'm not sure how this works. Thanks in advance.